Sometime the computer doesn’t see the external usb hard drive because of bad usb port. Remove the unnecessary device from working usb port and plug-in the hard drive in it. If the problems remains same then perform the following steps.
1. Go to Start
2. Select My Computer
3. Find Organize.
4. Select Folder and search options.
5. Now click on View tab.
6. Uncheck “Hide empty drives in the Computer folder”.
7. At the end click on OK.